Abstract
Abstract. Carbonyl sulfide (COS) has been suggested as a useful tracer for gross primary production as it is taken up by plants in a similar way as CO2. To explore and verify the application of this novel tracer, it is highly desired to develop the ability to perform continuous and high-precision in situ atmospheric measurements of COS and CO2. In this study we have tested a quantum cascade laser spectrometer (QCLS) for its suitability to obtain accurate and high-precision measurements of COS and CO2. The instrument is capable of simultaneously measuring COS, CO2, CO and H2O after including a weak CO absorption line in the extended wavelength range. An optimal background and calibration strategy was developed based on laboratory tests to ensure accurate field measurements. We have derived water vapor correction factors based on a set of laboratory experiments and found that for COS the interference associated with a water absorption line can dominate over the effect of dilution. This interference can be solved mathematically by fitting the COS spectral line separately from the H2O spectral line. Furthermore, we improved the temperature stability of the QCLS by isolating it in an enclosed box and actively cooling its electronics with the same thermoelectric chiller used to cool the laser. The QCLS was deployed at the Lutjewad atmospheric monitoring station (60 m; 6°21′ E, 53°24′ N; 1 m a.s.l.) in the Netherlands from July 2014 to April 2015. The QCLS measurements of independent working standards while deployed in the field showed a mean difference with the assigned cylinder value within 3.3 ppt COS, 0.05 ppm for CO2 and 1.7 ppb for CO over a period of 35 days. The different contributions to uncertainty in measurements of COS, CO2 and CO were summarized and the overall uncertainty was determined to be 7.5 ppt for COS, 0.23 ppm for CO2 and 3.3 ppb for CO for 1-minute data. A comparison of in situ QCLS measurements with those from concurrently filled flasks that were subsequently measured by the QCLS showed a difference of −9.7 ± 4.6 ppt for COS. Comparison of the QCLS with a cavity ring-down spectrometer showed a difference of 0.12 ± 0.77 ppm for CO2 and −0.9 ± 3.8 ppb for CO.
Highlights
Carbonyl sulfide (COS) has been suggested as a potential tracer for photosynthetic CO2 uptake (Sandoval-Soto et al, 2005; Montzka et al, 2007; Campbell et al, 2008; Berry et al, 2013; Asaf et al, 2013), as it follows the same uptake pathway into plants through stomata as CO2 but is not generally re-emitted by plants (Protoschill-Krebs and Kesselmeier, 1992; Protoschill-Krebs et al, 1996; Stimler et al, 2010b)
Measurements of reference standards in the field showed that concentrations drift on the order of 100 ppt (COS), 2 ppm (CO2) and 10 ppb (CO) per degree change of the electronics temperature in a few hours when the instrument was placed in an enclosed box and when the electronics section was actively cooled
We developed a means to analyze flasks; this allowed a comparison between the quantum cascade laser spectrometer (QCLS) and gas chromatographic mass spectrometry (GC-MS) measurements from the NOAA Boulder laboratory (Montzka et al, 2007)
Summary
Carbonyl sulfide (COS) has been suggested as a potential tracer for photosynthetic CO2 uptake (Sandoval-Soto et al, 2005; Montzka et al, 2007; Campbell et al, 2008; Berry et al, 2013; Asaf et al, 2013), as it follows the same uptake pathway into plants through stomata as CO2 but is not generally re-emitted by plants (Protoschill-Krebs and Kesselmeier, 1992; Protoschill-Krebs et al, 1996; Stimler et al, 2010b). When we tested the required frequency of background measurements we found that COS concentrations can shift to another level, either up or down, uncorrelated with instrument parameters such as temperature, and even when the background measurements were done shortly after each other (every 10 min). To test the frequency of reference measurements that is needed to remove drift due to temperature changes, we measured cylinder air over a period of 19 h, alternating with a reference gas every 15 min. Based on these results we can say that reference measurements every 30 min are sufficient to remove drift within 5.3 ppt with temperature changes up to at least 0.06 ◦C per 30 min. The effect of improved temperature stability was tested during field operation of the QCLS at the Lutjewad station (see Sect. 2.5)
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